Nowadays, portable electronic products become smaller and lighter, which has accordingly decreased the volume and capacity of their batteries. As a result, the efficiency of power supply modules needs to be improved. Furthermore, the power supply modules need to provide a stable output voltage when the battery voltage varies in a wide range, so as to improve the endurance of the batteries. Buck-boost converters which can work in a wide input range are widely used in these applications.
FIG. 1 illustrates a traditional buck-boost converter with four switches. The buck-boost converter converts an input voltage VIN into an output voltage VOUT. It comprises switches S1˜S4, an inductor L and an output capacitor C. Energy is stored in the inductor L when the switches S1, S3 are turned on and the switches S2, S4 are turned off. The energy stored in the inductor L is provided to a load when the switches S1, S3 are turned off and the switches S2, S4 are turned on. Since the switches S1˜S4 keep switching during the operation, the power loss of the traditional buck-boost converter is large.
To reduce the power loss, different working modes may be utilized according to the relationship between the input voltage VIN and the output voltage VOUT. When the input voltage VIN is smaller than the output voltage VOUT, the buck-boost converter works in a BOOST mode. The switch S1 is maintained on and the switch S2 is maintained off. The switches S3 and S4 are controlled through pulse width modulation. When the input voltage VIN is larger than the output voltage VOUT, the buck-boost converter works in a BUCK mode. The switch S4 is maintained on and the switch S3 is maintained off. The switches S1 and S2 are controlled through pulse width modulation.
In the method mentioned above, since the working mode is determined by the relationship between the input voltage and the output voltage, the relationship between the control loop and the feedback loop is complicated, which brings difficulties to system design and test. Furthermore, circuit parameters (such as voltage, current and duty cycle) suffer from sudden changes during the mode transition, which may induce spike on the output voltage.